logging in with user's email address only with Facebook account

hi I'm making an application which let user login with their Facebook account.

then after i get their profile such as email address, I store the email address to user table.

but I don't know how to log the user in.

here is my code below.

$fbUser = FacebookService::Instance()->getProfile();
$fbUser = json_decode($fbUser);
if(is_null($fbUser->email)){
    $this->Flash->error('We cannot retrieve your email address from Facebook');
}
$user = UserService::Instance()->getUserByEmail($fbUser->email);
session_destroy();
if($this->Auth->setUser($user->toArray())){
    $this->Flash->success('Login success with facebook');
}else{
    $this->Flash->error('login failed with facebook');
}
$this->set(compact('user'));

after initialize these code, I get login failed with Facebook error.

Please let me know Thank you

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